1. Field of the Invention
This invention relates to a rubber composition and a rubber article for pharmaceutical chemicals or medical treatments, obtained by forming and cross-linking this rubber composition, the rubber article having very excellent properties which can be adapted to the standard test values of various official documents, required in this field, and can further satisfy the test items having lately become an important problem from the pharmaceutical standpoint, and the rubber article further having good economical property.
2. Description of the Prior Art
As materials of containers for pharmaceuticals or devices for medical treatment, there have lately been used plastics in addition to glasses having been used from old times, and rubber articles have often been used as sealing materials thereof. High grade sanitary property and safety are required of the rubber articles in this field from the standpoint of influences on the human body. To this end, various test methods or quality standards are provided, for example, Test method of a rubber stopper for liquor transfusion or a plastic container for liquor transfusion of "11th Revision, Japanese Pharmacopoeia" (hereinafter referred to as JP 11), Standards of Medical Appliances such as "Appliance Standards for Artificial Kidney of Dialytic Type" according to Official Notification No. 494 of the Ministry of Health and Welfare, and those provided by the International Standards Organization (ISO), European Pharmacopoeia (EP), US Pharmacopoeia XXI (USP), West Germany Industrial Standard DIN 58, 366-58, 368 (DIN), British Standard 3263 (BS), etc. Furthermore, test items corresponding to the higher grade standards have lately been taken into consideration.
As these sealing materials, there have been proposed vulcanized isoprene-isobutylene rubbers (hereinafter referred to as IIR), rubber stoppers of isoprene rubbers (hereinafter referred to as IR) blended with polyethylene fine powder (Japanese Patent Publication No. 8789/1970), vulcanized butadiene rubber (hereinafter refrred to as BR) (Japanese Patent Publication No. 16409/1978), a rubber composition comprising an ethylene-propylene-non-conjugated diene ternary copolymer rubber (hereinafter referred to as EPDM) blended with zinc oxide, capable of passing Official Notification No. 434 of the Ministry of Health and Welfare (Japanese Patent Laid-Open Publication No. 157842/1983), a composition for a medical container using natural rubber (Japanese Patent Laid-Open Publication No. 99143/1985), etc. Above all, IIR is preferred because of its non-permeability of gases such as oxygen, nitrigen, etc, and moisture and its chemical inactivity, i.e. stability to acids, alkalies or chemicals.
Vulcanization of IIR has hitherto been carried out by the use of a cross-linking agent such as of sulfur or thiuram type, metal dithiocarbamates, quinoids, etc., or by resin cross-linking with phenol resins, etc. A rubber stopper consisting of the material obtained according to such cross-linking methods is not preferred because of contaminating pharmaceuticals through elution from the rubber stopper.
Thus, a zinc oxide-free rubber article for pharmaceuticals or for medical treatment and an improved one are proposed so as to remove, in particular, zinc of physiologically harmful chemical eluates (Japanese Patent Laid-Open Publication Nos. 183858/1982 and 216058/1983).
Cross-linking of IIR with organic peroxides is difficult because of causing collapsing or depolymerization, as well known in the art.
As compounds known from old times as organic peroxide cross-linking agents for rubbers or resins, there are dicumyl peroxide (DCP), benzoyl peroxide (BPO), etc. An example of using DCP for cross-linking IIR has been reported [1 Oxley and Wilson, "Rubber Chemistry and Technology" Vol. 42, p.1147-1154 (1964), 2 Imoto, Minoura and others, "Nippon Rubber Kyokaishi" Vol. 41, p.1004-1011 (1968)] and an example of using jointly DCP and maleimide compounds [3 Tawney and Wenisch, "Rubber Chemistry and Technology" Vol. 38, p.352-366 (1965)] has also been reported. However, rubber articles which can be put to practical use have not been obtained. Thereafter, it is only reported that cross-linking of IIR with organic peroxides is impossible.
Furthermore, methods have been proposed which comprise reacting and modifying ing IIR with chlorine, bromine or divinylbenzene to obtain a chlorinated butyl rubber (CIIR), brominated butyl rubber (BIIR) or divinylbenzene-modified butyl rubber (DIIR), which is then cross-linked with an organic peroxide ((Japanese Patent Publication Nos. 37824/1985, 46420/1975, 43865/1985), but the rubber articles obtained thereby are used as rubber vibration insulators and are not satisfactory, as a sanitary rubber article, in respect of chemical and physical properties.
The inventors have proposed sanitary rubber articles obtained by cross-linking rubbers each consisting of a mixture of a butyl rubber with a synthetic rubber of diene type or polyethylene, in Japanese Patent Laid-Open Publication No. 133347/1977 and further sanitary rubber articles each consisting of a mixture of IIR, CIIR or BIIR with fine powder of polyethylene of superhigh molecular weight, in Japanese Patent Laid-Open Publication No. 144346/1985.
Bonding methods comprising cross-linking IIR with organic peroxides and maleimides at a low temperature have been proposed in Japanese Patent Laid-Open Publication Nos. 90546/1977, 92343/1985, 130665/1985 and 130668/1985, but these methods have also disadvantages that the rubber article obtained therefrom is not satisfactory as a sanitary article because of tendency of discoloration, containing organic metal salts and inorganic metal salts and having a large problem on physical properties, e.g. compressive strain and heat resistance. In addition, there is no example of using a maleimide group-containing compound for forming a rubber article for pharmaceuticals or for medical treatment.
The articles of the prior art, described in the above described publications, are suitable for use as a sanitary article in some respects, but are not satisfactory in other respects. The reasons therefor probably consist in that cross-linking and shaping of IIR is very difficult. If this problem can be solved, therefore, it is expected to obtain a rubber article having physical and chemical properties suitable for use in the field of the present invention.